Verbal forms from the root KBS and the expression amr al-kabs indicate: (1) the intercalation and the use of the leap year in the Christian calendars (Biruni, ???r, pp. 32-33, p. 241); (2) the embolismal month of Jewish and Indian calendars; (3) the intercalation of the pre-Islamic Arab calendar calculated by a hereditary group termed qal?mes (Biruni, Tafhim, pp. 224-25; on the epithet, cf. “al-?alammas,” in EI ² IV, p. 472); (4) the periodical intercalation of a day in the Islamic lunar calendar (Biruni, Tafhim, p. 223); (5) the 5 days of Epagomenae of the Iranian and Coptic calendar (Biruni, Q?nun, pp. 74-76; Moll? Mo?affar, ?ar?-e bist b?b, b?b 2, section 3), in this way being in Iranian context a synonym of ?amsa-ye mostaraqa and andarg?h; (6) several readjustments that occurred in the Iranian solar calendar (Nowruzn?ma, pp. 11-12) up to the kabisa-ye malek??hi, that is, the leap of 18 days ordered by the Saljuq sultan Malek??h (r. 1072-92) to initiate the Jal?li era (Moll? Mo?affar, ?ar?-e bist b?b, b?b 2, section 4; see “The Jalali calendar” under CALENDARS ii). This last usage relates particularly to the readjustments for shifting Andarg?h along the year of the “Zoroastrian” calendar, but it does not indicate the deplacement of Andarg?h that occurred in the Iranian Yazdegerdi solar calendar in 1006 CE (Biruni, Q?nun, p. 76; the same Biruni, Q?nun, p. 142, referring to the reform proposed by the Abbasid caliph, states that “some people call the kabisa of al-Mo?ta?ed [r. 892-902] the kabisa of the Persians”). (7) The term is also applied to the calendar change performed by Parsi Indian communities in the 12th century (so among modern scholars; Coorlawalla, 1918).

The usual translation of kabisa as “intercalation,” that is, insertion of day(s) tout court, is therefore inadequate. Muslim scholars giving notices on the kabisa of the Persians were conditioned by two axioms: (1) the idea of Arabic kabisa (on which see A. Moberg, “Nas??,” in EI2 VII, 1993, pp. 977-78) as duplication in turn of all months (Biruni, ???r, pp. 62-63); (2) the Zoroastrian religious unacceptability of forced insertion of one day (cf. Biruni, Tafhim, p. 222). From these, perhaps, derives the theory of an entire one-month insertion every 120 years duplicating the whole of the months in turn. The first source dealing with the kabisa of the Persians in these terms, along with the shifting of Andarg?h, is the Zij al-j?me? by Ku?y?r b. Labb?n al-Jili (Ideler, pp. 547-48 tr.; pp. 624-25 text). This idea has been subsequently accepted by many astronomers, among them Biruni (???r, p. 11, p. 44). This pure mathematical-astronomical theory of intercalations is devoid of documentary evidence, but attracted the mathematicians in particular (de Blois, p. 40). In modern times it has been developed, firstly by A. von Gutschmid (pp. 1-9), into the theory of the double Iranian calendar (“religious” and “civil”; Taqiz?da, particularly pp. 13-16, 231).

Some sources later than Ku?y?r and less exploited deal with practices of a different kind, which possibly clarify the question. While the idea and the formula of an ‘intercalary’ turn persist, they refer, not to any insertion, but to mere shifting of Andarg?h to serve as a sort of indication of imminent Nowruz, the beginning of the year. According to the Zij al-mofrad (11th century) by Abu Ja?far Mo?ammad b. Ayub al-??seb al-?abari (Ms. O.1.10 Browne of the Cambridge University Library, fol. 3a): “The calculation (?om?r) of kabisa by the Persians in antiquity was a month in every 120 years, and now that practice has been left behind. . . . Their practice was the following: when the Sun reached the First Point of Aries in the first month of Dey, they called this month ??ar-m?h and ?b?n the month of ??ar. And the five stolen days [scil. Andarg?h] were counted at the end of [this] month of ?b?n. . . .” For this author the kabisa of the Persians implied only a shifting of Andarg?h along with a ‘renaming’ of all the months (Cristoforetti, pp. 46-47). According to Zakariy?? al-Qazvini (?Aj?yeb al-ma?luq?t, p. 82): “There exists divergence on this, some people asserting that the ?amsa al-a?ira [scil. Andarg?h] pertains to ab?n-m?h and some people asserting that the ?amsa al-a?ira pertains to a?ar-m?h.”